1. Field of the Invention
The present invention generally related to a method of manufacturing a semiconductor device and a semiconductor device manufactured thereby, and more particularly, to a semiconductor device manufacturing method wherein a well is formed in a semiconductor substrate by high-energy ion implantation.
2. Description of the Background Art
A semiconductor manufacturing method--by which, a well is formed by high-energy ion implantation after an isolating oxide film for separating element regions has been formed--is described in, e.g., (1) Japanese Patent Application Laid-open No. 6-252354 and (2) Japanese Patent Application Laid-open No. 5-74729. More specifically, reference (1) discloses a method comprising the steps of forming an isolating oxide film; forming a resist mask having an opening on a silicon nitride film used for forming the isolating oxide film as well as on the isolating oxide film, by omission of formation of a thermal oxide film commonly called a pad film; and forming a well by implanting ions through the opening. Reference (2) discloses a method comprising the steps of: forming a doped polysilicon cover film in place of a pad oxide film; implanting ions into a substrate through an opening formed in a resist mask; and subjecting the substrate to heat treatment for 6 hrs. at 1200.degree. C., thus forming a well.
The pad film plays a role in preventing introduction of undesirable impurities into a substrate from a resist mask or from outside the substrate at the time of ion implantation or heat treatment following the ion implantation. When high-energy ions pass through the pad oxide film, there occurs a knock-on phenomenon in which oxygen atoms are knocked on from the oxide film. Because of this phenomenon, there arises a problem of electrical properties of an element formed on a well being deteriorated by excessive oxygen atoms mixed into the surface of the substrate or by crystalline imperfections in a monocrystalline area within the substrate.
According to the technique described in reference (1), it is necessary to change implantation energy several times in a stepped manner in order to form a uniform well. Further, since the technique uses a silicon nitride film which is as thick as 100 nm, a great noncontiguous region arises along a boundary area between the silicon nitride film and the isolating oxide film. For this reason, the technique is not suitable for formation of a well of a microminiature LSI. Further, the reference does not at all describe knocked-on oxygen atoms. The technique described in reference (2) is intended to solve the problem stemming from the pad oxide film, through use of the cover film. However, the technique suffers a drawback of a substrate being subjected to heat treatment for a long period of time at high temperature.
Recent LSI chips are made compact and have their structure further complicated. For this reason, there exists a strong demand for both a reduction in thermal load on a semiconductor device throughout the entire manufacturing process and an improvement in reliability of the device so as to ensure a longer life of the device.
The present invention has been conceived to solve the aforementioned problems, and a first object of the present invention is to provide a semiconductor device manufacturing method which enables a considerable reduction in the amount of oxygen atoms included in a gate oxide film formed on a well or a considerable reduction in the degree of crystalline imperfection.
A second object of the present invention is to provide a semiconductor device manufacturing method which enables a recovery or improvement in the life of a gate oxide film of an element formed on a well through heat treatment at a comparatively low temperature.
A third object of the present invention is to provide an optimum combination of transistor structural parameters, conditions for ion implantation, and conditions for heat treatment, which is required for achievement of a recovery or improvement in the life of a gate oxide film.